Embodiments described herein relate to a method for injecting an emission liquid reductant into a gas stream, and more particularly, to a method for injecting ammonia into an exhaust gas stream of a diesel engine.
Diesel engine combustion results in the formation of nitrogen oxides, (NOx), in the exhaust gas. Aftertreatment systems, typically selective catalytic reduction systems (SCR systems) are used to reduce oxides of Nitrogen (NOx) emitted from engines. Nitrogen oxides can be reduced by ammonia (NH3), yielding N2, H2O and CO2. In the aftertreatment process, NOx reacts with the ammonia, which is injected into the exhaust gas stream upstream of an SCR Catalyst.
The aftertreatment process controls the ammonia injecting rate. An insufficient injection may result in low NOx conversions, and an injection rate that is too high may result in release of ammonia to the atmosphere, known as ammonia slip. The ammonia slip increases at higher NH3/NOx ratios. The ammonia injection depends on the injection characteristics and the energy of the exhaust gas. If the ammonia is improperly dosed with respect to the amount, the timing, the temperature, the space velocity, the current storage level and the NOx mass flow, ammonia slippage may result.
When the diesel particulate filter (DPF) of the aftertreatment system is in an active regeneration mode, the exhaust temperature and flow rate increase with respect to base operation. In this situation, an increased amount of ammonia is dosed, which depletes the ammonia stored on the DPF.
Further, for an aftertreatment system that uses solid ammonia, the quantity of ammonia stored on the vehicle should be sufficient to last the service life of the vehicle, while at the same time the quantity of ammonia should be nearly depleted at the end of the service life of the vehicle. Thus, the injection of ammonia should be sufficient to reduce the NOx, and should also correspond to the storage capabilities of the SCR system.